FIG. 1 illustrates a conventional switching power supply control system that utilizes pulse width modulation (PWM). A power converter 10 controls the flow of power from a power source VCC to the load 12 in response to a pulse width modulation signal PWM. The power converter includes a single switching transistor Q1, inductor L1, and diode D1 arranged in a classic buck configuration. An error amplifier 16 generates an error signal VERR by amplifying the difference between a feedback signal FB, which provides some measure of the output, and an input control signal VIN. A controller 14 generates a PWM signal that controls the switching transistor in the power converter in response to VERR, so as to provide a regulated output, typically a constant voltage, to the load.
FIG. 2 illustrates an example waveform of the PWM signal generated by the controller 14. At time t0, the PWM signal switches to the ON state which turns on the switching transistor in the power converter 10. A time t1, the PWM signal switches to the OFF state, thereby turning the switching transistor off. The pulse width is the amount of time from t0 to t1 during which the PWM signal is in the ON state. The waveform of FIG. 2 has a period T which is the time between successive ON transitions of the PWM signal. To control the amount of power to the load, the controller varies the pulse width (duty cycle) of the PWM signal by varying the time t1 at which the PWM signal switches off. For example, if the amount of power being supplied to the load is too low, the controller may leave the switch on longer until t1′ so that the transistor switch in the power converter is on for a longer portion of the switching cycle. Conversely, if the amount of power being supplied to the load is too low, the controller may leave the switch on only until t1″ so that the switch is on for a shorter portion of the switching cycle. The on time may be varied during each switching cycle to provide continuous output regulation.
The example described above may be referred to as trailing edge modulation because the second switching event (the turn off event in this case) during each switching cycle is varied. In leading edge modulation, the first switching even (e.g., turn on time) is varied, while the PWM signal turns off at a fixed time.
Various techniques have been suggested for improving the transient response of PWM switching power supplies. One technique involves the use of a voltage controlled oscillator (VCO) to control the switching cycle of the PWM signal. By applying an error signal to the VCO, the switching frequency of the PWM signal may be changed dynamically in an attempt to maintain a constant output voltage into a changing load. Another approach referred to as a glitch catcher circumvents the normal control loop to provide a temporary output current pulse directly from the supply VCC into the load in an attempt to prevent the output voltage from falling in response to a sudden increase in the load.